1. Field of the Invention
The present invention relates to an improvement in a compensation control device for a power system that compensates the reactance of the power system, controls the reactance, controls an eddy current or controls stabilization.
2. Description of the Related Art
A conventional compensation control device for a power system will be described with reference to FIGS. 8A and 8B. FIG. 8A is a diagram showing the conventional compensation control device for a power system having a switched capacitor disclosed in Article No. 530-02 reported in CIGRE symposium relating to the power field in Tokyo on May 22 to 24, 1995. FIG. 8B is a diagram of a CSC (controlled series compensator) disclosed in Article No. 210-06 reported in that symposium.
In FIG. 1 of the former article, there is shown a plurality of switched capacitors, each of which is turned on/off by thyristor switches which are disposed for switching a capacitive reactance that influences a line current. The swithes turn on/off a plurality of capacitors connected in series and employ thyristor switches connected in parallel with those capacitors, respectively.
In FIG. 4, 3-1 of the latter article, there is shown a CSC in which a fixed series capacitors and a reactor which is thyristor-controlled. The capacitors are connected in parallel to sequentially control the energization angle of the thyristor, thereby sequentially controlling the composite reactance with the capacitors.
In FIG. 8A of this disclosure, each of switched capacitors 100 includes a capacitor 101, a thyristor switch 102, a capacitor short-circuit current limit reactor 103, and an arrester 104. Also, as shown in FIG. 8B, a CSC 500 includes a fixed capacitor 501, an energization angle controlling thyristor 502, an energization angle controlled reactor 503 and an arrester 504.
The switched capacitor 100 allows a capacitive reactance to be changed step by step, and the CSC 500 allows the capacitive reactance to be continuously changed. The combination of them allows a total capacitive reactance to be continuously changed. This variable reactance function makes it possible to realize impedance compensation of a transmission line, reactive power compensation due to a series compensation, and their control. Also, those controls make it possible to conduct the stabilization control of a power system, the adjustment of an eddy current, the adjustment of a line impedance, the adjustment of phase difference between both ends, etc.
However, the CSC 500 that realizes the continuous control function suffers from problems stated below. In a state where the capacitive reactance of the CSC 500 is the smallest, the thyristor 502 is non-energized. A circuit diagram in this state is shown in FIG. 9A.
The capacitive reactance of the CSC 500 in this state becomes a fixed value Xco of the capacitor 501, and the entire line current I flows in the fixed capacitor 501 whereas no current flows in the thyristor 502 and the reactor 503.
Then, the energization angle of the thyristor 502 is increased so that the reactor current flows. Since the reactor current is opposite in phase to the current flowing in the capacitor 501, when the polarity of the reactor current is reversed, the fundamental wave component of the reactor current is identical in phase with the line current, and a sum of the reactor current and the line current flows in the capacitor 501, to thereby raise a voltage across the capacitor 501.
In other words, the susceptance of the thyristor-controlled reactor 503 is canceled by the susceptance of the capacitor 501, to thereby increase the composite capacitive reactance.
As a result, a relation between an energization angle .alpha. of the thyristor 502 and the composite capacitive reactance Xc is varied as represented by a curve of FIG. 9C. If the maximum energization angle is 180.degree., the composite capacitive reactance Xc becomes KXco. That is, the reactance Xc becomes K times as large as the minimum value. A circuit in this state is shown in FIG. 9B. In this case, KXco=1/{(1/Xco)-(1/X.sub.L)} is satisfied, where X.sub.L is the reactance of the reactor 503.
Therefore, the reactance of the reactor 503 required for controlling the capacitive reactance in a range of Xco to KXco becomes X.sub.L =KXco/(K-1).
In the conventional compensation control device for a power system, in the above state, the voltage of the capacitor 501 in the CSC 500 becomes K times, and the VA capacitance of the capacitor 501 becomes K.sup.2 times, as large as the VA capacitance Qco in a state shown in FIG. 9A. Also, the VA capacitance of the reactor 503 becomes (K-1) KQco. However, since the composite capacitive reactance is KXco, the composite capacitive reactive power is only KQco. In other words, because the reactive power effectively exerted on the line is increased K times, there arises a problem that a capacitor 501 of K.sup.2 times and a reactor 503 of K(K-1) times are necessary.